During the production of semiconductors, residues and defects have to be removed from a semiconductor wafer. To remove residues, an industry proven solution called Standard Clean-1 (SC-1) is used. The SC-1 solution comprises deionized (DI) water, ammonium hydroxide (NH4OH) or ammonia (NH3) and hydrogen peroxide (H2O2). A typical concentration of such an SC-1 solution is 50 parts by volume of DI water, 2 parts by volume of hydrogen peroxide and 1 part by volume of ammonium hydroxide. The SC-1 solution is either used exclusively or in combination with megasonic energy to facilitate defectivity reduction.
During the SC-1 clean, etch of an oxide underneath the defect and the presence of electrostatic repulsion between the residue and the wafer facilitates the detachment of the residue from the wafer into the bulk solution where it can be filtered out. When megasonic energy is used in combination with the SC-1 solution, the need for strong oxide underetch to dislodge the residues is minimized since the mechanical action of the megasonic energy supports this function. Therefore, the SC-1 solution in combination with megasonic energy can be significantly diluted to minimize oxide etch thereby minimizing negative impact on devices being fabricated as well as to reduce the costs of the cleaning chemistry.
The use of megasonic waves or megasonic cleaning refers to a cleaning technique where forces released during high frequency cavitation bubble collapse are used to overcome the adhesion forces between the substrate to be cleaned and the undesirable contaminants adhering to it. During the application of megasonic waves, cavitation is induced by cycling the pressure signal being transmitted through the regimes conducive to bubble nucleation and bubble implosion. The frequency of the impressed pressure signal controls the maximum cavitation bubble size. In general, higher megasonic frequency leads to smaller cavitation bubble size and vice versa.
The cleaning efficiency at a particular particle size is a function of the sonic frequency. FIG. 1 shows the relation between particle size and particle removal rate. The cleaning efficiency for cleaning smaller size particles increases with higher sonic frequency. A stagnant liquid boundary layer thickness near the substrate surface reduces as the megasonic frequency is increased. This is illustrated in FIG. 2. For effective particle cleaning, the boundary layer should be smaller than the particles desired to be removed (J. Harman, E. L. Lamm, “The impact of ultrasonic frequency on particle removal”, Branson Ultrasonic Corp.).
At constant temperature, if a liquid is subjected to decreasing pressure, the liquid undergoes a phase change when the pressure falls below the vapor pressure of the liquid. Nucleation of the cavitation bubbles in the bulk liquid in the absence of contaminants or solid surfaces is termed homogenous nucleation while nucleation at interfaces is termed heterogeneous nucleation. From theoretical considerations relating to homogenous nucleation it is known that higher megasonic power enhanced by liquid pressure amplitude leads to a higher potential for substrate damage. On the other hand, a reduction of megasonic power also reduces the velocity of microjets which could potentially contribute to damaging sensitive structures on the wafer. For a theoretical background see C. E. Brennen, “Cavitation and Bubble Dynamics”, Oxford University Press, 1995.
Hence, if the megasonic power during the cleaning step could be reduced at a similar or even increased cleaning efficiency, the potential damage to structures on the wafer can be reduced.